Battery packs are known in which a plurality of storage batteries (for example, a plurality of lithium ion secondary battery cells) are connected.
Patent Document 1 discloses a battery pack system having a plurality of series cell units in which a plurality of storage batteries are connected in a series, the plurality of series cell units being connected in parallel.
In the battery pack in which series cell units (storage battery units) are connected in parallel, the problems arise that variations in voltage among the series cell units cause cross currents to flow from series cell units having high voltage to series cell units having low voltage during charging. A reduction of variations in voltage among the series cell units is therefore to be desired.
In the battery pack system described in Patent Document 1, current control elements that can each adjust the magnitude of current that flows through its own current control element, for example, MOSFET (Metal Oxide Semiconductor Field Effect Transistors), are each connected in series to a respective series cell unit, and by using each current control element to control the magnitude of current that flows to each series cell unit, variations in the voltage among the series cell units are reduced.
The current control element disclosed in Patent Document 1 is an element that adjusts the magnitude of current that flows through its own element by adjusting its own resistance value.